1. Field of the Invention
The present invention relates to determining navigation heading or attitude using a satellite based positioning system and antenna array.
2. Background Information
Commonly used attitude determination apparatus and methods include use of geomagnetic sensors, gyroscopic sensors or phase differences of signals received from multiple satellites of the Global Navigation Satellite System (GNSS) or other similar systems.
Geo-magnetic sensors are susceptible to errors from locally generated magnetic fields (from nearby electronics) and from nearby magnetic objects. Additionally, local earth magnetic fields must be mapped and available to the user. Gyroscopic sensors drift and must be re-calibrated regularly. Such limitations have hindered the use of these devices.
GNSS systems and techniques, including differential techniques, are now commonplace and as background are only briefly described herein. Signals from GNSS satellites contain information used by a GNSS receiver to determines a satellite's orbit, altitude, location and speed, and information that allows the GNSS receiver to determine its location on Earth and its distance from a satellite. If four satellites are sending data the position of the receiver can be calculated accurately. If fewer satellite signals are being received the receiver's position can be still be found or approximated.
Prior art measuring of phase differences of GNSS signals requires accurate measurements of the carrier phase measurements between antennas at known spacings. Such measurements are prone to errors due to multi-path effects even though there are antennas and algorithms to counter these effects. But, such antennas and algorithms are costly and typically block out low elevation satellites thus reducing the number of satellites used for the measurements.
Another limitation of prior art techniques using differential phase GNSS measurements is that long baselines (long times) are needed for accurate headings, but longer baselines make acquisition more difficult, and shorter baselines reduce accuracy. For example, a prior art 50 cm baseline may provide a one degree accuracy, whereas in a preferred embodiment of the present invention a 19 cm baseline provides a one degree accuracy—more than a 40% improvement.
Others have used GNSS phase differences, for example, U.S. Pat. No. 6,128,557 to Fenton et al., and which is commonly owned with the present invention, issued Oct. 3, 2000. This patent describes an invention that determines position and attitude of a rotating space vehicle using GNSS techniques. A phase discriminator measures phase differences that are used with signal strength, Doppler shifts to produce a “spin signature” that is used to calculate the space vehicle spin rates and attitude. Another U.S. patent, U.S. Pat. No. 6,018,315 to Ince et al., owned by Motorola, describes a system using differential measurements for attitude sensing. However, both of these inventions are subject to the limitations described above.